freebsd man page for sockatmark

SOCKATMARK(3)						   BSD Library Functions Manual 					     SOCKATMARK(3)

NAME
     sockatmark -- determine whether the read pointer is at the OOB mark

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <sys/socket.h>

     int
     sockatmark(int s);

DESCRIPTION
     To find out if the read pointer is currently pointing at the mark in the data stream, the sockatmark() function is provided.  If sockatmark()
     returns 1, the next read will return data after the mark.	Otherwise (assuming out of band data has arrived), the next read will provide data
     sent by the client prior to transmission of the out of band signal.  The routine used in the remote login process to flush output on receipt
     of an interrupt or quit signal is shown below.  It reads the normal data up to the mark (to discard it), then reads the out-of-band byte.

	   #include <sys/socket.h>
	   ...
	   oob()
	   {
		   int out = FWRITE, mark;
		   char waste[BUFSIZ];

		   /* flush local terminal output */
		   ioctl(1, TIOCFLUSH, (char *)&out);
		   for (;;) {
			   if ((mark = sockatmark(rem)) < 0) {
				   perror("sockatmark");
				   break;
			   }
			   if (mark)
				   break;
			   (void) read(rem, waste, sizeof (waste));
		   }
		   if (recv(rem, &mark, 1, MSG_OOB) < 0) {
			   perror("recv");
			   ...
		   }
		   ...
	   }

RETURN VALUES
     Upon successful completion, the sockatmark() function returns the value 1 if the read pointer is pointing at the OOB mark, 0 if it is not.
     Otherwise the value -1 is returned and the global variable errno is set to indicate the error.

ERRORS
     The sockatmark() call fails if:

     [EBADF]		The s argument is not a valid descriptor.

     [ENOTTY]		The s argument is a descriptor for a file, not a socket.

SEE ALSO
     recv(2), send(2)

HISTORY
     The sockatmark() function was introduced by IEEE Std 1003.1-2001 (``POSIX.1''), to standardize the historical SIOCATMARK ioctl(2).  The
     ENOTTY error instead of the usual ENOTSOCK is to match the historical behavior of SIOCATMARK.

BSD
								 October 13, 2002							       BSD